A phenomenon known in the art of acoustics is that of the echo. During a musical performance, a listener will detect sound waves coming directly from the musician, or a loudspeaker, and will also receive sound waves which have been reflected from an object, such as the wall or ceiling of an auditorium. The reflected sound is typically delayed by a small period of time and contributes to the overall sensation by the listener.
In an attempt to recreate this sensation, it is known to electronically delay a part of the signal driving a loudspeaker so that the speaker essentially broadcasts the direct sound wave and a simulation of the reflected sound wave. The delayed signal is produced by an electronic delay line.
In addition to the desire to recreate the echo effect, modern musicians use a variety of delay times to create particular types of music.
Many of the prior techniques utilize analog circuits, but the music industry is rapidly turning to digital electronic techniques for amplification of audio signals. A typical digital audio system directs an input to an analog-to-digital converter (ADC) followed by a digital signal processor and a digital-to-analog converter (DAC), the output of which is connected to a speaker.
It is known that a delay of a digital signal can be produced by storing the digital signal in a memory and reading the signal out of the memory at a later time. For example, if the recall address is shifted from the store address by a number of storage elements, the delay time will be equal to the product of the sample rate and the difference between the store address and the recall address. The delay length in accordance with this technique is typically varied by altering the sample rate.
U.S. Pat. No. 4,005,268 (Canell et al.) shows a solid-state echo producing system wherein an analog input signal is converted to a digital signal, and the digital signal is then directed to a series of shift registers. The signal is read out of the shift registers and converted back to an analog signal. The shift registers are controlled by a clock of variable frequency to alter the length of the delay.
Systems such as those shown in Cannell et al. have extremely limited delay variability and require input and output filtering to vary also in accordance with the changing sample rate in order to obtain optimum performance of the system.
Another type of digital delay system employs storage and recall address generators which are separately controllable. U.S. Pat. No. 4,181,975 (Jenkins) shows a digital delay line wherein an input signal is stored in a memory in accordance with a clock rate. The signal is read out of the storage medium by a recall address generator controlled by a voltage controlled oscillator (VCO). The recall address generator frequency is determined by a comparison of the store and recall addresses to produce a phase signal which is used as the input to the VCO.